Perturbed Rydberg series of autoionizing resonances

Three-channel quantum-defect theory (3QDT) provides a simple and transparent description of how perturbations affect energies, widths, and photoabsorption line shapes in a Rydberg series of autoionizing resonances. As long as the resonances in the perturbed series do not overlap, very simple formulas can be derived for the positions, widths, and line-shape parameters without any approximation other than neglecting the energy dependence of the 3QDT parameters. We illustrate the applicability of the formulas in the example of a hydrogen atom in a magnetic field, where we present first ab initio calculations of photoabsorption spectra in the regime of field strengths where Rydberg series of resonances belonging to different Landau channels interfere.